1. Field of the Invention
The present invention relates generally to the field of electric furnaces. More particularly, it relates to an electric furnace having a rotatably mounted crucible inside an enclosure.
2. Description of the Prior Art
Electric induction furnaces are used, among other purposes, for melting metals which have very high melting points, such as titanium which becomes molten at a temperature of approximately 3000.degree. F. In casting such metals, it is very important that the temperature be accurately controlled and variations should, typically, not exceed 15.degree. F.
An induction furnace typically comprises an enclosure having an access door. Through one wall of the enclosure, which may be the access door, a crucible is mounted for containing the metal after it has become molten. The electric induction coils surround the crucible and are internally water cooled to prevent melting. The space within the enclosure is maintained under vacuum and means are provided to tilt the crucible in order to pour the molten metal therefrom, frequently directly into appropriately positioned molds.
To supply the required electric current and cooling water to the induction coils of the crucible, flexible water cooled power cables were heretofore attached to the coils with unions. To enable the required rotational movement of the crucible so that it can be tilted for pouring molten metal therefrom, the hoses were given sufficient lengths so that they could be twisted upon the tilting of the crucible.
This arrangement has a number of drawbacks. First, because of the heavy current carried in the hoses, they are relatively stiff. As a result, the unions coupling the hoses to the coils are subjected to relatively heavy stresses and they have a tendency to work loose. This can result in unacceptable water leakage. Thus, it has been necessary to constantly inspect and retighten the unions and, if they become worn, to replace them with new ones. This is cumbersome, time-consuming and expensive.
A more serious drawback is that each time the crucible is tilted, the relative position of the conductive hoses changes. Such changes in their relative position changes the electric power coupling to the induction coils which changes the induction of the entire system. This, in turn, changes the temperature that is achieved within the crucible for a given power setting. Consequently, with prior art constructions, it was difficult to maintain the required close temperature tolerances within the crucible, which are necessary for high quality melting, and sophisticated and expensive temperature controls had to be provided.
A further shortcoming of the prior art arrangements discussed above is the fact that the angle through which the crucible can be tilted is relatively limited by the ridgidity of the conductive hoses. Although it was possible to tilt the crucible through more than 90.degree. from its vertical melting position into an inclined pouring position, tilting the crucible beyond a total of 120.degree. to 130.degree. was difficult, caused increasingly large forces on the induction coils and the coil-hose unions and, under extreme circumstances, could result in damage to the crucible and induction coils. This is a critical problem because the crucible is normally constructed of low strength material, such as asbestos, refractory brick and the like.